How do you add error handling to this? Obviously, I could change the
signature to return an Either but I don't see how I "return" anything
other than Right.

E.g., I would want to give a warning like "Expected 'a', or 'b'" if
anything other than 'a', or 'b' is used as first character. Similarly
for '1' and '2'. The obvious(?) way would be to add an error result as
the last option. The best I can come up with is adding ReadP.pfail to
the list of choices but that doesn't allow me to add a useful error.
Is ReadP simply not powerful enough for this? Or am I overlooking
something obvious?

I did some googling and there is plenty about creating parsers with
parser combinators but I could not find anything about error handling.

Re: ReadP & Error Reporting

My first instinct is that no, there's not much you can do with this. You could wrap every combinator to return more detailed error messages, which might be *okay*. Or you could use the heavier parsing libraries and get all the error reporting for free.

How do you add error handling to this? Obviously, I could change the
signature to return an Either but I don't see how I "return" anything
other than Right.

E.g., I would want to give a warning like "Expected 'a', or 'b'" if
anything other than 'a', or 'b' is used as first character. Similarly
for '1' and '2'. The obvious(?) way would be to add an error result as
the last option. The best I can come up with is adding ReadP.pfail to
the list of choices but that doesn't allow me to add a useful error.
Is ReadP simply not powerful enough for this? Or am I overlooking
something obvious?

I did some googling and there is plenty about creating parsers with
parser combinators but I could not find anything about error handling.